Purge Sulfate Solids

FIGURE 6-4. Wellman-Lord process. (From Elliot, T. C., Ed., Standard Handbook of Powerplant Engineering, McGraw-Hill, New York, 1989. With permission.)

same time. The overall regeneration reaction is:

2NaHSO3(ag) + heat Na2SO3(s) + H2O(l) + SO2(conc.) (6-31)

The concentrated SO2 stream that is produced may be compressed, liquefied, and oxidized to produce sulfuric acid or reduced to elemental sulfur. A small portion of collected SO2 oxidizes to the sulfate form and is converted in a crystallizer to sodium sulfate solids that are marketed as salt cakes [8].

The advantages of this process include minimal solid waste production, low alkaline reagent consumption, and the use of a slurry rather than a solution, which prevents scaling and allows the production of a marketable by-product. Disadvantages of the process are the high energy consumption and high maintenance due to the complexity of the process, as well as the large area required for the system. Another disadvantage is that a purge stream of about 15% of the scrubbing solution is required to prevent build-up of the sodium sulfate. Thiosulfate must be purged from the regenerated sodium sulfite.

Regenerative Magnesia Scrubbing In the magnesium oxide process, MgO in the slurry is used in a manner similar to the use of limestone or lime in the lime scrubbing process. The primary difference between the processes is that the magnesium oxide process is regenerative, whereas lime scrubbing is generally a throwaway process.

Hot Air Reheat

Primary Particulate Collector I (baghouse or ESP) Prescrubber

^ To Stack

Primary Particulate Collector I (baghouse or ESP) Prescrubber

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